Nullable Column with Constraint Error on Insert

I have a table which has a nullable column (ie, Null is allowed), but which also has a foreign key constraint. During an insert, I am getting an error when I leave out the fourth column, which I presume would make the column null:

Could not execute 'INSERT INTO ATTRIBUTE_VALUE (ATTR_VALUE_KEY, ATTR_KEY, VALUE) values (30,1,'VALUE 30-1')' in 96 ms 467 µs . SAP DBTech JDBC: [461]: row locked by other transaction:

However, if I were to add the a key for the fourth column, DATA_SOURCE_SYSTEM_KEY, referencing a valid row, the insert will work.

Why would this happen?